Yes, silnylon will soak through when in contact with anything wet. This was the reason I quit using the sil bags in my pack. Plastic sheets do not, but they are a real pain in the butt to get dried out. As Tipi mentioned, the poly tarps from Walmart work about the best, out of the box. But, they can be a bit heavy and are volume intensive in your pack. Once you cut them down, they tend to unravel easily at the edges unless you use a rolled type hem (double stitched,) too. Fortunatly, duct tape can be used to prevent further fraying, but the whole deal will weigh around 8-10oz.
The best compromise solution I’ve found is simply to coat your existing floor material with mineral spirits and silicone…old style mineral spirits (or white gas in colder weather) and clear silicone caulk mixed in about a 10:1 to 15:1 ratio. Brushed on to both sides, it will pretty much seal the nylon against leaking when in contact with water/snow. It’ll take about 24 hours to set…in cold weather as long as 72 hours. After you are done, it can still breath a slight amount of water vapour, but this is different, and not real leakage, more of condensation related stuff. Anyway, it will add about an ounce to a heavily coated ground cloth and not impede packing size/volume except for slightly. It will also provide a bit of texture and stop the slipperiness of normal silnylon. Anyway, 2 yards should be plenty and it shouldn’t weigh more than 4-4.5oz when completed. Not Ultralight, but in winter conditions, it works well.
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